As a liquid level sensor which detects a liquid level of an engine oil or a fuel in an automobile, a construction machine, or the like, a liquid level sensor as shown in FIG. 33 or 34 is known (see Patent Document 1).
FIG. 33 is a front view of a detecting unit of a conventional liquid level sensor. In FIG. 33, first comb-shaped detecting electrode unit 2 is arranged on a lower end portion of rectangular substrate 1 which vertically extends. Second comb-shaped detecting electrode unit 3 is arranged on a central portion of substrate 1.
First detecting electrode unit 2 is configured by a plurality of linear electrodes 4 vertically arranged at predetermined intervals. Linear electrodes 4 are alternately connected to extraction lines 5 and 6 which vertically extend along both side edges of substrate 1.
Second detecting electrode unit 3 is configured by a plurality of linear electrodes 7 which are arranged to extend from an upper end portion to a lower end portion at predetermined intervals from side to side. Linear electrodes 7 have upper ends alternately connected to extraction lines 8 and 9.
In liquid level measurement, the detecting unit is dipped in a liquid to be measured. More specifically, first detecting electrode unit 2 is arranged to be always dipped in the liquid to be measured. On the other hand, second detecting electrode unit 3 crosses to the liquid level to be measured, and a part dipped in the liquid increases or decreases in size with rising and falling of the liquid level.
FIG. 34 is a detecting circuit diagram of a conventional liquid level sensor. In FIG. 34, a detecting circuit is configured by oscillating circuit 10 and processing circuit 17. Oscillating circuit 10 has inverters 11, 12, and 13 and resistor 14. First and second detecting electrode units 2 and 3 which configure the detecting unit in FIG. 33 are connected between inverters 12 and 13 through analog switches 15 and 16.
Processing circuit 17 having a microcomputer closes analog switch 15 first calculates a dielectric constant of the liquid to be measured from an oscillation frequency determined by resistor 14 and a capacitance of first detecting electrode unit 2 to store the dielectric constant. Processing circuit 17 closes other analog switch 16 in place of analog switch 15. A liquid level is calculated based on an oscillation frequency determined by resistor 14 and a capacitance of second detecting electrode unit 3 and the dielectric constant of the liquid to be measured.
As information of prior art document related to the invention of the application, for example, Patent Document 1 is known.
However, in the conventional liquid level sensor, a dielectric constant of a liquid to be measured is calculated from an oscillation frequency of oscillating circuit 10 determined by a capacitance between the electrodes of first detecting electrode unit 2 always dipped in the liquid to be measured and the dielectric constant is stored. Thereafter, the level of the liquid to be measured is calculated from the oscillation frequency of oscillating circuit 10 determined by a capacitance between the electrodes of second detecting electrode unit 3 crossing to the liquid level to be measured and having a part which is dipped in the liquid and increases or decreases in size with rising and falling of the liquid level, and the dielectric constant of the liquid to be measured. Therefore, an operational device to calculate the liquid level becomes complex and has a large scale.